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Department of Agriculture & Ecology

Lars Stoumann Jensen & Sander Bruun

lsj@life.ku.dk

Soil carbon sequestration with biochar from waste - the potential of thermo-chemical conversion technologies

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New buzzwords in a climate struck world…

”Bioenergy” … ”Biomass-fuels””CO2-neutral energy” … ”Carbon footprint”

But many derived problems…Whilst in some cases being more CO2 neutral thanfossil fuels, bioenergy may often

• compete directly with food production• divert biomass carbon from recycling to soil• lead to land use change with huge soil C

emissions as a consequence

Soil carbon sequestration with biocharrepresents a potential solution

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Kyoto article 3.4 (LULUCF additional activities)

• How much does soil C mean in Denmark?• 157 t C/ha in farmed topsoil (0-100 cm avg.) • 2,7 mio. ha farmed area = 1550 mio. t CO2-equiv.• Danish Kyoto reduction target:

70 mio. t CO2-equiv. emitted * 21 % reduction= 15 mio. t CO2-equiv.

• Total Danish CO2 target may be reached with just a relative increase in soil C by 1 % per year!

• Denmark has chosen to adopt Kyoto-protocol article 3.4• Relatively few other countries have adopted 3.4• Significant documentation monitoring and verification required• Once adopted, no step back!• Need for new soil C sequestration mechanisms

if reduction target is not met!

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Development of carbon stocks in Danish mineral soils (modelled)

(Gyldenkærne, Petersen og Olesen, 2007, MSt AR 5)

Ban on straw field burning

Ref. year 1990:∆C=-1,54 mio t C/y(5 y avg.)

Commitment period 2008-12:∆C=0,35 mio t C/y

Net change: 1,9 mio t C/year extra sequesteredLarge CO2 quota value for Denmark!

”Warm years”

Predicted with1961-90 climate

Avg. climate (1961-90)Actual climateC

arbo

nin

min

eral

soi

ls(m

io. t

C)

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Development of carbon stocks in Danish mineral soils (modelled)

(Gyldenkærne, Petersen og Olesen, 2007, MSt AR 5)

ift. klima i 2001-2004

ift. klima i 1961-1990ift. klima i 1961-1990

Most likelyscenario acc. to IPCC

However:• Increasing temperature may jeopardise article 3.4 gains!• Bioenergy will remove more C inputs = less soil C seq. • Need for technology alternatives, producing both

bioenergy and soil C sequestration

Car

bon

in m

iner

al s

oils

(mio

. t C

)

(actual climate)

Temperature:Avg. 1961-90+2 ºC/100y rel. to 1961-90+3 ºC/100y rel. to 1961-90+2 ºC/100y rel. to 2001-04

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Bioenergy with biochar – net CO2 removal?

• Bioenergy with biochar can result in a net removal of CO2 from the atmosphere, but only if• The biomass is biogenic• A significant net energy output is produced• The biochar is recalcitrant and highly stabilised in soil

(Sohi et al 2009)

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Pyrolysis and biochar from plant residues

PlantsSoil

Pyrolysis

Nutrients

Biochar

Plant resid

ues

Bio-Oil

Gas

C sequestrationSoil quality

Energy

CO2

CO2 50%

CO2 only 5%

100%

• International biochar initiative (IBI) promotes biochar as an official UNFCCC mitigation-strategy / Clean Development Mechanism (CDM)

• Discussed at COP14 in Poland and to be discussed in Copenhagen Dec 2009 at several COP15 side events

50%

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Conditions:T = 700 –1200 CAt the prese of Oand water steam

o

nce 2

Chemical conversions

BIO-OIL

GAS

FEEDSTOCK PRE-TREATMENT

Chopping indingrying (45 C)

& GrD o

XRDNRIGC/MSTGASANMRFTIR16SrDNA

ANALYSIS

SOIL AMENDMENT

Conditions:T = 350 – 650 C

Without Oo

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GasificationPyrolysisTHERMAL CONVERSION

Catalytical gasification

BIOSYNGASCO, H2

Chemicalmodification

Chemical industry

FEEDSTOCKAgricultural wastesSewage sludgesBiorefinery wastesDedicated plantations

Chemicals industry

Electricity

FT diesel

BIOCHAR

A more sustainable Carbon cycle

(W. Kwapinski, www.carbolea.ul.ie)

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More flexible outputs…

(W. Kwapinski, www.carbolea.ul.ie)

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PYROLYSIS400 - 600ºC

no O2

CONDENSATION

CATALYTIC CONVERSIONTO HYDROGEN (Optional)

Vapors

GasesH , CO, CH , C H2 4 2 4

Biochar Heat

Biomass LiquidsPOWERGENERATION

CHEMICALSEPARATION

or

COMBUSTION

Advantages of Pyrolysis:generally a simple,low-cost technology,capable of processing a wide variety of feedstocks,offers the unique advantage of giving a: liquid-fuel that can be stored and transported, Bio-char, gas-fuel.

Biomass liquefaction via pyrolysis

(W. Kwapinski, www.carbolea.ul.ie)

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Table: Typical product yields (dry wood basis) obtained by different modes of pyrolysis of wood

Mode Conditions Liquid Gas Char

Fast pyrolysis moderate temperature, around 500°C, short hot vapour residence time ~1 s 75 % 13 % 12 %

Intermediate pyrolysis

moderate temperature, around 500°C, moderate hot vapour residence time ~10-20 s 50 % 30 % 20 %

Slow pyrolysis (carbonisation)

low temperature, around 400°C, very long solids residence time 30 % 35 % 35 %

Gasification high temperature, around 800°C, long vapour residence time 5 % 85 % 10 %

75 %

35 %

85 %

Yield of Pyrolysis

(W. Kwapinski, www.carbolea.ul.ie) GreenCarbon & Stirling

pyrolysis CHP plant, Denmark

Dynamotiveplant

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What is biochar and its effects?• Biochar is the solid that remain after light gases

and tar have been released from the biomass• Charcoal like, high carbon content• Positive effects on soil quality and plant

productivity:• Increase surface area and porosity = higher water

and nutrient adsorptive capacity• Improved filtering of pollutants• Increase activity of autotrophic and symbiotic

microorganisms = increased crop productivity

• Extremely slowly degradable in soil = a way of sequestering biomass derived carbon

Fungal growth

Porous structure

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The soil carbon challenge – depletion of soil humus, especially in S. Europe

• Organic matter in soils(humus) is essential for soilfertility and influencesressource use efficiency(water, energy) and CO2storage (C sequestration)

• Worldwide soil humus levels are being depleted, due to deforestation, intensive cropping, removalof biomass for bioenergy

• In Europe especially the Mediterranean soils are lowin humus

• Biochar may serve as an important organicmatter input

(EU-JRC Soil Atlas)

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The soil carbon challenge – also on certain soils in Denmark

• Dexter Ratio = clay / org. C has been proposed as an indicator of soil C saturation

• Low values indicate soilsdepleted in complexed C

• In Denmark especially loamysoils, low in humus areidentified as depleted

• Dominant in eastern parts with few animals, close to metropolitan areas withlots of organic MSW

(Schønning et al. 2009)

OK Problem

Seriousproblem!

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00,10,20,30,40,50,60,70,80,9

1

0 100 200 300 400 500 600 700

Days of soil incubation

Frac

tion

of b

iom

ass

C re

mai

ng

00,10,20,30,40,50,60,70,80,9

1

0 100 200 300 400 500 600 700

Days of soil incubation

Frac

tion

of b

iom

ass

C re

mai

ngStability of biochar in soil

Bruun et al. (2008)

Plant material (straw)

Biochar prod. from straw at 225ºC

Biochar prod. from straw at 300ºC

Fraction of biomass C remaining

Biocharmore resitantto decaythanrawbiomass

(incl. loss during pyrolysis)

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Biochar stability and effects on decomposition- investigated using 14C labelling

Soilorganicmatter

Plant residues Biochar

CO214C

14C

14C (40 y ago)

14CO2

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Biochar degradability in soil- depends on degree of thermal alteration

Bruun et al. (2008)

dried plant mat.

pyrolysed

Respiration of plant material / biochar (14C-labelled)

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Decomposition of plant residues in soil is not affected by biochar application

Respiration from straw

00.050.1

0.150.2

0.250.3

0.350.4

0.450.5

0 50 100 150 200 250 300 350

Day

Frac

tion

of s

traw

C r

espi

red

Plant residues in soil with:no biochar addedbiochar added

(14C-labelled)

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Decomposition of biochar is not affected by plant residue addition

Respiration from biochar

00.0020.0040.0060.0080.01

0.0120.0140.0160.018

0 50 100 150 200 250 300 350

Day

Frac

tion

of b

ioch

ar C

re

spire

d

Biochar in soil with:no straw addedstraw added

(14C-labelled)

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Decomposition of soil organic matter (SOM)is not affected by biochar addition

Respiration of SOM

0

0.01

0.02

0.03

0.04

0.05

0.06

0 50 100 150 200 250 300 350

Day

Frac

tion

of S

OM

resp

ired

00.40% straw0.15% char0.77% char1.55% char

(soil humus 14C-labelled 40 y ago)

Soil onlyEquivalentamount of plant mat.

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Conclusions and final remarks

• Soil C sequestration has significant potential for climate change mitigation – but may bejeopardised by global warming

• New and more efficient waste bioenergytechnologies needed, which combine energyproduction and soil C sequestration

• Gasification / pyrolysis technology withbiochar production has significant potential

• Biochar recalcitrance depends on degree of thermal alteration

• Biochar addition to soil does not alter decomposition of other added organicresidues or old soil humus

• Still need for more system analyses in order to assess sustainability